Merciful spell + sneak attack

[Rickmeister]

Another marvelous question, I find myself in dubio about...

Character: Ro3/Wi2
Abilities: Sneak attack +2D6, touch (attack) spells

As far as I know, the rogue can deliver a sneak attack through a touch spell.
He can also sneak-attack for non-lethal, with a sap...
He can also merciful spell for non-lethal, with a spell...

Catch my drift?
Can my (player's) Ro/Wi use a non-lethal spell, and add his sneak-attack, while doing *only* non-lethal damage?

Copied from PRD:

Spoiler:

Merciful Spell (Metamagic)
Your damaging spells subdue rather than kill.
Benefit: You can alter spells that inflict damage to inflict nonlethal damage instead. Spells that inflict damage of a particular type (such as fire) inflict nonlethal damage of that same type. A merciful spell does not use up a higher-level spell slot than the spell's actual level.

Sneak attack
With a weapon that deals nonlethal damage (like a sap, whip, or an unarmed strike), a rogue can make a sneak attack that deals nonlethal damage instead of lethal damage. She cannot use a weapon that deals lethal damage to deal nonlethal damage in a sneak attack, not even with the usual –4 penalty.

[Ascalaphus]

Yeah, the "weapon" (the spell) deals non-lethal, so the player can choose to deal non-lethal sneak attack damage. Seems pretty clear to me.

As a GM I'd applaud the player; non-lethal attacks open up interesting situations.

[Umbranus]

RAW I'd say the sneak attack deals lethal damage. As far as I know only a sap has the rule about dealing non-lethal sneak attack.
But I'd houserule it to be optional.

[Rickmeister]

So far, RAW it's "Sneak is lethal", because 'spell' differs from 'weapon'.
RAI is a different story..

How about a ray? It qualifies for weapon specialization, no?